The present invention relates to apparatus and methods for acoustic logging in earth formation around a borehole.
An important parameter to be determined in logging an oilfield earth formation around a borehole is the velocity (or its inverse: xe2x80x9cslownessxe2x80x9d) of acoustic propagation in the formation. Determining velocity or slowness effectively involves measuring the time taken by an acoustic signal to travel a known distance in the formation at the borehole wall, typically using an acoustic transmitter and a plurality of acoustic receivers. To minimize the effects of spurious signals, the transmitter is typically excited by periodic pulses.
Acoustic logging of velocity in an earth formation can be performed in a previously drilled borehole or while drilling the borehole. Conventional logging in a previously drilled borehole is known as xe2x80x9cwireline loggingxe2x80x9d. In wireline logging, an acoustic logging tool is lowered into and then pulled out of, a previously drilled borehole, on an armored xe2x80x9cwirelinexe2x80x9d communication cable. Conventional logging while drilling the borehole is known as xe2x80x9clogging while drillingxe2x80x9d (xe2x80x9cLWDxe2x80x9d). In LWD an acoustic logging tool is attached to a thick steel mandrill close behind the drill bit.
Conventional acoustic tools used in wireline logging or LWD typically include one transducer configured as a transmitter to generate and transmit an acoustic signal, and a plurality of transducers, configured as an array of receivers, that detect the acoustic signal in the borehole. These transducers can be made of piezoelectric ceramic materials which, when used as transmitters, expand and contract transversely to their surfaces (i.e., change in thickness) in response to electrical excitation, or conversely, when used as receivers, generate electrical voltages between those surfaces when subjected to pressure fluctuations. In the case of transmitters, they can also be electrodynamic. That is, an electrodynamic assembly can drive their transducing operation in a manner that is, in principle, similar to the electrodynamic assemblies used in loudspeakers in many radios and stereo systems. Thus, the transmitter can be driven with an appropriate oscillating electrical signal to generate pressure fluctuations in the liquid in the borehole.
These pressure fluctuations travel (xe2x80x9cpropagatexe2x80x9d) as acoustic signals through the liquid and into and through the surrounding formation. Some of the acoustic signals propagating through the formation couple back into the borehole liquid to produce electrical voltage signals at the outputs of the receivers. These voltage signals are sensed, amplified, and processed downhole to extract information for transmission up the cable. Alternatively, the waveforms of the voltage signals can be transmitted uphole, for example as digitized time samples, for processing at the surface.
The type of transmitter most commonly used, a cylinder, generates compressional pressure waves (xe2x80x9cP wavesxe2x80x9d) in the borehole liquid. However, acoustic logging tools are not limited to investigating the propagation of P waves. When the P wave reaches the borehole wall, some of the acoustic energy is typically converted into other modes of acoustic propagation. Thus, both P waves and shear waves (xe2x80x9cS wavesxe2x80x9d) can be excited in the formation. The P waves can also excite guided borehole modes in the borehole. Guided borehole modes include the monopole Stoneley mode (the lowest radial order monopole borehole mode), the dipole flexural mode (the lowest radial order dipole borehole mode), and the quadrupole screw mode (the lowest radial order quadrupole borehole mode). They can also excite sextupole (also called hexapole) and other higher azimuthal order borehole modes. The P waves can also excite higher radial order monopole, dipole, and quadrupole modes in the borehole. The relative level of excitation for each of the modes depends on such factors as the transmitter type, the formation type, the borehole size, the frequency range, and how well centered the transmitter and/or the tool is, amongst other factors. Because these different modes generally travel at different speeds, they can sometimes be distinguished in the receiver signals. Determination of such parameters as the speed and attenuation of P,S, Stoneley, and flexural waves is useful in investigating a variety of subsurface formation properties of interest in the exploration for hydrocarbons and other valuable raw materials.
Prior art sonic borehole logging tools typically use the monopole and dipole formation and lowest radial order borehole modes (Stoneley and flexural) for acoustic logging. Prior art sonic borehole logging tools that use the dipole mode typically measure the sound field downhole in a borehole using an axial array of dipole receivers. Such tools are discussed in U.S. Pat. No. 4,951,267 (the Chang patent), U.S. Pat. Nos. 3,593,255, 4,446,544, and 5,343,001. If too many types of borehole modes and/or formation waves are significantly present in the pressure signals sensed at the receivers, then identifying specific acoustic pulses in the receiver signals and selecting the desired pulse becomes difficult. Thus, a major challenge in the design of acoustic logging tools that use the dipole mode, is to significantly improve the rejection of monopole and other non-dipole azimuthal (multipole) mode contamination that can be simultaneously generated and propagated in the borehole. A similar challenge is presented in the design of acoustic logging tools that use the quadrupole mode, or the sextupole mode, or any other multipole mode.
Differentiation between monopole and dipole in acoustic logging in a borehole is typically accomplished in the prior art by using a receiver having an xe2x80x9cazimuthal arrayxe2x80x9d which has only two receiver elements that face each other.
The prior art includes various kinds of multipole or azimuthally asymmetric transducers suspended in the borehole liquid for direct or indirect shear wave logging. The prior art includes dipole transducers as disclosed in U.S. Pat. Nos. 3,593,255; 4,207,961; 4,383,591; and 4,516,228, and GB patent specifications 2,124,377 and 0,031,989. It further includes quadrupole transducers as disclosed in GB patent specifications 2,122,351 and 2,132,763. It further includes octapole transducers as disclosed in GB patent specification U.S. Pat. No. 2,130,725. U.S. Pat. No. 4,369,506 discloses the use of geophones suspended in the liquid in a borehole, the geophones buoyancy-adjusted to be nearly neutral to encourage sympathetic movement with the borehole wall. U.S. Pat. No. 4,542,487 discloses buoyancy-adjusted geophones in orthogonally mounted pairs. These buoyancy adjustments are difficult to make accurately and significantly complicate the use of such devices.
Another type of dipole receiver, a xe2x80x9cbenderxe2x80x9d, includes a pair of oppositely polarized piezoelectric plates securely joined together. Such receivers are described in U.S. Pat. Nos. 4,516,228; 4,606,014; 4,649,525; 4,774,693; 4,782,910 and 5,357,481.
Transmitters for producing quadrupole and sextupole (hexapole) modes in a borehole are described in U.S. Pat. Nos. 4,855,963 and 4,932,003. These transmitters include a thin-walled, radially poled, piezoelectric cylinder divided electrically into an even number of closely matched segments.
The invention provides a method for detecting a received acoustic pulse of a selected azimuthal borehole mode in a liquid-containing borehole in a formation. The method uses a sonde having an axial array of acoustic receiver stations aligned with the borehole, each receiver station having an azimuthal array of at least four piezoelectric receiver elements, the receiver elements uniformly spaced apart around the azimuthal array. An acoustic pulse transmitted into the formation produces an electrical signal at each receiver element. Azimuthal spatial filtering is applied to a representation of the pulse after the pulse has passed through the formation to produce data representing pressure associated with the selected azimuthal borehole mode. The number of the at least four piezoelectric receiver elements in a receiver station is at least equal to, and is generally greater than, the minimum number of azimuthally arrayed receiver elements required to resolve pulses of the selected mode without aliasing.
In a first preferred embodiment of the method, analog to digital conversion is performed on an electrical signal from a receiver element to produce element data representing the electrical signal. This is repeated for other receiver elements in the azimuthal array to produce element data for each of the other receiver elements. Azimuthal spatial DFT filtering is applied, according to the selected mode, to element data of a receiver station to produce receiver station data for that receiver station. This is repeated for each receiver station of the axial array. Then, using receiver station data, a received acoustic pulse of the selected azimuthal borehole mode is detected.
In a second preferred embodiment of the method, electrical signals from apodized receiver elements are combined to produce element data representing a cosine transform of the pressure associated with the receiver station.
Electrical signals from apodized receiver segments are appropriately summed together to produce data associated with cosine apodized receiver elements. For the dipole case, element data is combined by differencing signals from pairs of apodized receiver elements. For the general multipole case, element data is combined by summing signals from every other receiver element. Signals resulting from the resulting pair of receiver element groups are then differenced. These steps are repeated for each receiver station of the axial array to yield a set of receiver station electrical signals. The set of receiver station electrical signals is processed to produce data representing the received acoustic pulse of the selected azimuthal borehole mode.
The invention provides an apparatus for detecting a received acoustic pulse of a selected azimuthal borehole mode in a liquid-containing borehole. The apparatus includes a sonde having an axial array of acoustic receiver stations aligned with the borehole, each receiver station having an azimuthal array of piezoelectric receiver elements. It also includes a transmitter for transmitting an acoustic pulse into the formation, and a data processor, wherein at least either the data processor or the axial array includes means for applying azimuthal discrete spatial transform filtering. Each receiver station includes an azimuthal array of at least four piezoelectric receiver elements. The receiver elements are uniformly spaced apart around the azimuthal array. In a first preferred embodiment of the apparatus, the data processor includes software for applying azimuthal spatial discrete Fourier transform filtering digitally. In a second preferred embodiment of the apparatus, the axial array of acoustic receiver stations includes apodized receiver elements.